C/s-l,3-dimethylcyclohexane

A The diaxial conformation of c/s-l,3-dimethylcyclohexane is approximately 23 kj/mol (5.4 keal/mol) less stable than the diequatorial conformation. Draw the two possible chair conformations, and suggest a reason for the large energy difference. 

Probably the most interesting observation in Table 3.2 concerns the 1,3-dimethylcyclohexanes. Unlike the 1,2- and 1,4-dimethylcyclohexanes, in which the trans stereoisomer is more stable than the cis, we find that c s-l,3-dimethylcyclohexane is 7 kJ/mol (1.7 kcal/mol) more stable than ran5-l,3-dimethylcyclohexane. Why ... 

The steric interference between substituents in axial positions is particularly severe when there are large groups on two carbon atoms that bear a 1,3-diaxial relationship (cis on Cl and C3, or Cl and C5), as in the two chair conformations of c/s-l,3-dimethylcyclohexane shown here. The less stable conformation has both methyl groups in axial positions. The more stable conformation has both methyl groups in equatorial positions. Note the strongly unfavorable 1,3-diaxial interaction between the two methyl groups in the diaxial conformation. The molecule can relieve this 1,3-diaxial interference by flipping to the diequatorial conformation. Use your models to compare the diaxial and diequatorial forms of c/s-l,3-dimethylcyclohexane. 

D. C/s-l,3-dimethylcyclohexane in the diaxial conformation. Note the significant steric interaction between the 1,3-diaxial substituents. 

How many NMR absorptions would you expect for c/s-l,3 dimethyl-cyclohexane For fra/rs-l,3-dimethylcyclohexane Explain,... 

As revision, before proceeding, with the aid of models confirm that m-l,2-dirnethylcyclohexarse and c/.v-1,3-dirnethylcyc to hexane are nwstK and therefore achiral. Also make a model of both traus-l,2-dimethylcyclohexane and /w ,s-l,3-dimethy]cyclohexane and confirm that the molecules are chiral. 

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